A. Field of the Invention
This invention relates to novel copolymers of ethylene and hexafluoropropene (HFP) containing 15 mol percent to 50 mol percent HFP and methods for preparing the same. The copolymers of this invention are highly viscous oils having little tendency to flow at ambient temperatures and are characterized by having low to intermediate molecular weight. This invention relates further to the preparation of the copolymers of this invention by polymerizing ethylene and HFP in the presence of a source of free radicals under relatively mild reaction conditions in a medium comprised of a perfluoroalkane or perfluorochloroalkane liquid, or a mixture of one or more of said perhaloalkanes with water.
B. Description of the Prior Art
It is well known in the prior art that HFP cannot be homopolymerized in a free radical system without the use of prohibitively high pressures. In addition, the free radical initiated polymerization of ethylene requires high pressure (5,000 to 10,000 psi). The copolymerization or terpolymerization of HFP with other fluoroolefins; e.g. tetrafluoroethylene, vinylidene fluoride and chlorotrifluoroethylene is known and a number of these polymers have gained importance commercially; see for example U.S. Pat. No. 3,787,379. On the other hand, the copolymerization of HFP with non-fluorine-containing monomers has received little attention.
Copolymers of HFP with non-fluorine-containing olefins and vinyl monomers are disclosed in U.S. Pat. No. 2,549,935; in Example 4 of which, a monomer mixture containing approximately 4 mol percent HFP and 96 mol percent ethylene was polymerized in the presence of water at 70.degree. C. with lauroyl peroxide for 9.5 hours. A pressure in the range of 8,400 to 8,800 psi was maintained throughout the polymerization by injection of water resulting in an 8% yield of copolymer containing 10 mol percent HFP.
Adams et al., J. Polymer Sci., IX (6), 481-492 (1952) reported an HFP/ethylene copolymer containing 8 mol percent HFP obtained in 26% yield in a 4-hour emulsion polymerization with potassium persulfate at 80.degree. C. and 5100 psi. The copolymer was reported to be rubbery and weak as well as low in molecular weight.
The information available from U.S. Pat. No. 2,549,935 and the Adams et al. publication indicates that high pressures must be used in order to polymerize a mixture of ethylene and HFP, and even when such high pressures are utilized, low yields of such copolymers are obtained containing relatively low amounts of HFP incorporated therein. On the other hand, U.S. Pat. No. 3,817,951 discloses low-modulus thermoplastic ethylene/tetrafluoroethylene (TFE)/HFP terpolymers containing between 40 and 60 mol percent ethylene, between 20 and 30 mol percent TFE and between 10 and 30 mol percent HFP. The products of the present invention are distinguishable from U.S. Pat. No. 3,817,951 in several respects. In the first place, the polymers of the patent are terpolymers, whereas those of the present invention are copolymers. The polymers of the patent contain TFE, which, as is well known, can be homopolymerized in the presence of a free radical source, whereas HFP cannot be under the conditions of the patent. Moreover, the terpolymers of the reference are solids, whereas the copolymers of this invention are low to intermediate molecular weight oils having high viscosity. Furthermore, the patent does not contemplate a polymer containing more than 30 mol percent HFP.
Japanese Patent Publication J4 9083-789, published August 12, 1974, discloses copolymers containing ethylene and 11 to 50 mol percent HFP prepared at 90.degree. to 300.degree. C. at pressures of 800 to 4000 kg/square cm. (approximately 11000 to 57000 psi) to provide a vulcanizable product.